The present invention relates to a process for the manufacture of photopolymer plates having a highly constant thickness and high homogeneity, in which a mixture of a thermoplastic polymer, at least one ethylenically-unsaturated compound, a photoinitiator and, if appropriate, further solid and/or liquid additives is extruded and then calendered.
Photopolymer plates are used, for example, for producing relief plates for letterpress printing, particularly flexographic printing. The plates used for this purpose, which have thicknesses between about 0.5 and 7 mm, must have a highly precise and constant thickness in order to minimize the deviations occurring in the thickness of the thermoplastic material with respect to width and processing direction during the continuous manufacturing process. Apart from the constant thickness of the product, a reproducibly adjustable homogeneity of the composition of the thermoplastic materials is also essential.
U.K. Patent No. 1,358,062 and U.S. Pat. No. 4,423,135 disclose the preparation of light-sensitive layers comprising a thermoplastic polymer, an ethylenically-unsaturated compound, a photoinitiator and further additives by homogenizing the components in a mixing apparatus, for example, in an internal mixer, a roll mixer or a mill, in combination with melting of the polymer, and then forming photopolymer layers from this mixture in a separate processing step by extrusion, calendering or platen-press molding. In this process, a disadvantage is the batchwise procedure. Exposure of the material to thermoplastic stress, which takes place at least twice, can lead to a change in the mixing components; the result can be troublesome gel formation and nonhomogeneity.
EP-A 0,080,665 describes a process in which, in a self-cleaning twin-screw extruder, the mixing components are melted and homogenized and the melt is discharged through a sheet die into the gap of a two-roll calender with formation of a rotating bead and shaped to give photopolymer plates. It is said to be crucial for achieving the desired precision and constant thickness of the layer that a rotating bead comprised of the material to be processed be formed before the roll gap. The disadvantage of this process is that the gap opening of the sheet die has to be adapted in each specific instance to the desired thickness of the photopolymer layer leaving the calender roll gap. A further disadvantage is the direct feed of the melt from the twin-screw extruder into the sheet die. As is known, a twin-screw extruder is distinguished by batchwise delivery leading to pressure pulsations (cf., for example, "Der Extruder als Plastifiziereinheit" (The extruder as plasticizing unit), p. 114/115, VDI-Verlag, Dusseldorf 1977). The result is a non-uniform discharge of the melt into the calender roll gap, which has adverse effects on the consistency of the thickness of the photopolymer plates. A further disadvantage is the presence of a rolling bead in the calender gap, which may give rise to air being sucked into the thermoplastic material, which can have adverse effects on the transparency and shelf life of the product.